Title says it all. However, this is for humor only, just remember that.
I'll start.
...when you are told to go to your room, you reply "My movement stat is too low."
...when you suddenly become super-social with everyone to "try and get an A-Rank support with them"
...when you suggest names of FE characters for the drama club's play, and everyone on the Drama Club thinks you're a pro at making up awesome names.
...when you turn FE characters into D&D characters
...when you know the storyline of every FE game you've played better than you know your class schedule
...when you start saying "vulnerary" instead of "medicine"
That's all I can think of for now - feel free to add your own!